One way to detect the logic state of a switching device is to couple the device between a power source and ground and measuring the resulting voltage. For example, in FIG. 1A, power is applied at terminal 101, which is coupled in series with a resistor 102 and a switching device 104 to a ground 105. The switching device 104 may be a single switching device, such as a transistor, or a more complex device, such as a series of switching devices which form a logic circuit having a logic output. The logic state of the switching device 104 may be determined by measuring the voltage at terminal 103. If the voltage at terminal 103 is relatively high, then the switching device 104 is in a open state. Similarly, if the voltage at terminal 103 is relatively low, then the switching device 104 is in a closed state. The change in voltage at terminal 103 is related to the current flow rate through the switching device. Thus, the voltage sensing at terminal 103 should be performed only after the sufficient time has elapsed for the voltage to become stable after a state change in the switching device 104.
An issue which arises when using a circuit such as illustrated in FIG. 1A in a semiconductor device is that of parasitic capacitance. Parasitic capacitance is a unwanted capacitance resulting from the fabrication of the semiconductor device and is typically associated with conductive lines. FIG. 1B illustrates a circuit equivalent to that illustrated in FIG. 1A, but with the parasitic capacitance illustrated explicitly illustrated as capacitor 106 coupled in parallel to the switching device 104 in-between resistor 102 and ground 105. The effect of parasitic capacitance is to reduce the rate a voltage at node 103 changes over time as the switching device 104 switches states. For example, if the switching device 104 were open and then switched to a close position, the voltage a node 103 in FIG. 1B would fall towards its new value at a slower rate than if the parasitic capacitance 106 were not present. Parasitic capacitance, therefore, increases the time required to detect a changed state of the switching device 104.
One method for compensating the reduced switching speed imposed by parasitic capacitance is to provide increased current flow through the circuit. Increasing the maximum current flow through the switching device 104 discharges the charge stored by the parasitic capacitance faster when switch 104 is closed and changes capacitor 106 faster when switch 104 is opened. Thus, increasing the maximum current flow throughout the circuit permits the voltage at node 103 to reach a stable state faster after the switching device 104 has changed its logical state. Unfortunately, increasing the maximum current flow also increases the power consumption of the circuit. Accordingly, there is a need and desire for a method and apparatus to quickly and efficiently detect a logic state of a device in an environment having significant parasitic capacitance.